Multipath distortion is caused when two or more radio signals modulating the same message signal arrive at a receiver at different times and with different phases and attenuation levels due to reflections of the signal from various objects. The radio signal, prior to arriving at the receiver, can experience changes in amplitude and phase due to two types of multipath fading: flat and frequency-selective. In urban environments, reflections from close-in objects such as buildings result in short delay multi-path fading, causing wideband deep fades. In this scenario all spectral components of the signal experience amplitude attenuation simultaneously (flat fading). In turn, long multipath delay (or frequency selective fading) arises due to reflections from objects several kilometers away such as distant hills and tall buildings. In this case subsets of sub-carriers in the signal are impaired. Multipath impairments on a digital radio signal result in increased bit error rates at the demodulator output and may result in audio distortion and or partial loss of data services.
Antenna diversity is a scheme whereby two or more antennae are used to receive a common signal and the signal is combined to compensate for fading impairments. The fundamental assumption of receiving the same signal on spatially separated antennae and combining the signals is that of uncorrelated fading. That is, while the signal received on one antenna may be faded, little to no fading will occur on the signal received on the other antenna. Moreover, a diversity receiver can produce a signal with higher signal-to-noise ratio (SNR) (up to 3 dB) when no fading is present, resulting in greater audio quality and data services integrity as compared to a single antenna system. Although antenna phase diversity for analog FM broadcast signals is known and implemented in certain vehicle platforms, current architectures cannot handle digital radio broadcasts (for example HD™ radio or Digital Audio Broadcasting (DAB)).